Pollution control devices are used on motor vehicles to reduce atmospheric pollution. Two types of devices are currently in widespread use: catalytic converters and diesel particulate filters or traps. Catalytic converters contain one or more catalysts, which are typically coated on the surface of a monolithic structure. The monolithic structure is typically ceramic, although metal monoliths have been used. The catalyst(s) oxidize carbon monoxide and hydrocarbons or reduce the oxides of nitrogen in exhaust gases. Diesel particulate filters or traps typically are in the form of wall flow filters that have a honeycombed monolithic structure made from porous crystalline ceramic materials. In the current state-of-the-art construction of these pollution control devices, the monolithic structure of each type is enclosed within a metal housing.
Protective packing materials are typically positioned between the monolith and the metal housing to protect the monolith from road shock and vibration, to compensate for the thermal expansion difference between the metal housing and the monolith, and to prevent exhaust gases from passing between the monolith and the metal housing. The process of positioning the protective packing material is referred to as “canning” and includes such processes as injecting a paste into a gap between the monolith and the metal housing, or wrapping a sheet material (i.e., mounting mat) around the monolith and inserting the wrapped monolith into the housing, pressing the housing closed, and welding flanges along the lateral edges of the housing.
Typically, the compositions used to form conventional protective packing materials include glass or refractory ceramic fibers that provide properties such as high temperature durability, good handling, resiliency, flexibility, and strength. An intumescent material can also be included that enables the protective packing materials to volumetrically expand at elevated temperatures. Such expansion can help hold the monolith in place during use.